Anti-interference Problems of PLC Control System

Abstract: Based on the design, installation, use and maintenance experience in recent years, the author proposes that the green integration capability of PLC system must be improved and the problems that should be paid attention to in application.

1 Electromagnetic interference sources and interference to the system
1.1 General classification of interference sources and interference
The interference sources that affect PLC control systems are the same as those that affect industrial control equipment.
Most
of them are generated at locations where current or voltage changes dramatically. The locations where these charges move dramatically are noise sources, i.e., interference sources. Interference types are usually divided according to the causes of interference, noise interference modes, and the waveform properties of the noise. Among them, according to the causes of noise generation, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the waveform and properties of noise, it is divided into continuous noise, occasional noise, etc.; according to the noise interference mode, it is divided into common mode interference and differential mode interference. Common mode interference and differential mode interference are a relatively common classification method. Common mode interference is the potential difference between the signal and the ground. It is mainly caused by the common mode (same direction) voltage drop induced by the
power
grid series, ground potential difference, and space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially when the power supply is a distributor with poor isolation performance, the common mode voltage of the transmitter output signal is generally high, and some can be as high as 130 V or more. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, directly affecting the measurement and control signal, causing damage to components (this is the main reason for the high damage rate of UO modules in some systems). This common mode interference can be divided into DC and AC. Differential mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly formed by the coupling induction of the electromagnetic field between the signals and the voltage formed by the conversion of common mode interference by the unbalanced circuit. It is directly superimposed on the signal and directly affects the measurement and control accuracy. 1.2 The main sources of electromagnetic interference in PLC control systems 1.2.1 Radiated interference from space The radiated electromagnetic field in space is mainly generated by the transient process of power networks, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually called radiation interference, and its distribution is extremely complex. If the PLC system is placed in the radio frequency field, it will receive radiated interference, which is mainly affected by two paths: one is the direct radiation to the inside of the PLC, which is caused by the induction of the circuit; the other is the radiation to the PLC communication network, which is caused by the induction of the communication line. Radiated interference is related to the layout of the on-site equipment and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables and PLC local shielding and high-voltage discharge components. 1.2.2 Interference from the external leads of the system (1) Interference from the power supply. The normal power supply of the PLC system is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by all spatial electromagnetic interference and induce voltage and circuit on the line. In particular, changes in the power grid, switching operation surges, start-up and stop of large power equipment, harmonics caused by AC and DC transmission devices, and short-circuit state impacts of the power grid are all transmitted to the power supply through the transmission line. PLC power supplies usually use isolated power supplies. However, its structure and manufacturing process factors make its isolation performance not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible. (2) Interference introduced by signal lines. In addition to transmitting effective information, all kinds of signal transmission lines connected to the PLC control system will always be invaded by external interference signals. There are two main ways of interference: one is the interference of the power grid connected through the transmitter power supply or the power supply of the shared signal instrument, which is often ignored; the other is the interference of the signal line by the electromagnetic radiation induction in the space, that is, the external induction interference on the signal line, which is very serious. The interference introduced by the signal will cause the UO signal to work abnormally and the measurement accuracy to be greatly reduced. In severe cases, it will cause damage to components. For systems with poor isolation performance, it will also cause mutual interference between signals, causing the common ground system bus to reflux, resulting in changes in logical data, malfunctions and crashes. (3) Interference from chaotic grounding systems. Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the interference emitted by the equipment. Incorrect grounding will cause serious interference signals and make the system unable to work normally. 1.2.3 Interference from inside the PLC system Mainly caused by the mutual electromagnetic radiation between components and circuits inside the system, such as mutual radiation of logic circuits and their impact on analog circuits, the mutual impact of analog ground and logic ground, and the mismatch between components. These are all part of the electromagnetic compatibility design of the system by the PLC manufacturer, which is relatively complex and cannot be changed by the application department, but modules with more application performance or Ji Luzhang test should be selected. 2 Anti-interference design of PLC control system engineering application 2.1 Equipment selection When selecting equipment, first of all, products with high efficiency anti-interference ability should be selected, including electromagnetic compatibility. Especially the ability to resist external interference, such as the PLC system with cage isolation performance using floating ground technology: Secondly, the anti-interference indicators given by the manufacturer should be understood, such as common analog ratio, differential analog ratio, voltage resistance, and the allowed electric field strength and high frequency magnetic field strength environment; the other is to examine its application performance in similar work. When choosing imported products, please note that my country uses a 220 V high internal resistance power grid, while Europe and the United States use a 110 V low internal resistance power grid. Due to the large internal resistance of my country's power grid, large zero potential drift, and large ground potential changes, the electromagnetic interference on site of industrial enterprises is at least 4 times higher than that in Europe and the United States, and the system's anti-interference performance requirements are higher. PLC products that can work normally abroad may not be able to operate reliably in China. Therefore, when using foreign products, we must make reasonable choices according to my country's standards.

2.2 Comprehensive anti-interference design
Mainly consider the various suppression measures from outside the system. The main contents include: evaluate the PLC system and external leads to prevent electromagnetic interference from space radiation; isolate and filter the external leads, especially the power cables, and arrange them in layers to prevent the introduction of conducted electromagnetic interference through the external leads; correctly design the grounding points and grounding devices, and improve the grounding system. In addition, software means must be used to further improve the safety and reliability of the system.
3 Main anti-interference measures
3.1 Use a power supply with excellent performance to suppress the interference introduced by the power grid
In the PLC control system, the power supply occupies an extremely important position. The power grid interference is mainly introduced into the PLC control system through the power supply of the PLC system (such as CPU, power supply, I/O power supply, etc.), the power supply of the transmitter, and the power supply of the instrument with direct electrical connection to the PLC system. For the power supply of the transmitter and the common signal instrument, select a distributor with small distributed capacitance and large suppression band (such as multiple isolation and shielding and leakage inductance technology) to reduce the interference of the PLC system. In addition, to ensure that the power grid is not interrupted, an online uninterruptible power supply 9UPS0 can be used to improve the safety and reliability of power supply. And UPS also has strong interference isolation performance, which is an ideal power supply for PLC control systems.
3.2 Cable selection and laying
In order to reduce the electromagnetic interference radiated by power cables, especially the electromagnetic interference radiated by power cables of frequency converters. In the project, steel belt armored shielded power cables are used to reduce the electromagnetic interference produced by power cables. When wiring over long distances, the input signal line and the output signal line use their own cables respectively. The AC signal and the DC signal use their own cables respectively. The input and output signal lines are wired separately from the high voltage and high current power lines. The input and output signal lines of integrated circuits or transistor devices must use shielded cables. Avoid laying signal lines and power cables close to each other in parallel to reduce electromagnetic interference.
3.3 Qualitative analysis of I/O modules
Insulated input, output signals and internal circuits Insulated
anti-interference performance is good; bidirectional thyristor and transistor-type contactless outputs generate little interference on the PLC controller side; the input module allows a large input signal ON-OFF voltage difference and has good anti-interference performance; input modules with slow input signal response time have good anti-interference performance. Therefore, from the perspective of anti-interference, the following factors should be considered when selecting I/O modules: in situations with much interference, use insulated I/O modules; I/O modules installed on the control object side should use insulated I/O modules; in situations without external interference, non-insulated I/O modules can be used. 3.3.1 Anti-
input signal interference
In addition to using filters and good grounding of the controller to suppress interference, the following anti-input interference measures can also be considered. When there is an inductive load at the input end, in order to prevent the reverse impact induced electromotive force, a resistor and a capacitor are connected in parallel at both ends of the load (for AC input signals), see Figure 1, or a freewheeling diode is connected in parallel (for DC input signals), see Figure 2. When the input mode is alternating, the selection of resistors and capacitors must be appropriate to achieve better results. When the load capacity is below 1OVA, 0.1μF and 120Ω are generally selected; when the load capacity exceeds 1OVA, 0.47μF and 47Ω are more suitable. If the inductive load connected in parallel with the input signal is large, it is better to use a relay for transfer. Measures to prevent induced voltage K DA. Connect a surge absorber in parallel at the input end; ② In long-distance wiring and high-current situations, the induced voltage is large and can be converted by a relay.

3.3.2 Preventing interference of output signals
Output signal interference occurs: In the case of inductive load, a sudden current is generated when the output signal changes from OFF to ON, and a reverse induced electromotive force is generated when the output signal changes from ON to OFF. All of these may cause interference. Measures to prevent interference:
(1) For AC inductive load, connect R and C in parallel at both ends of the load as surge absorbers. When the AC 220V voltage power is about 400VA, the values ​​of R and C are 47Ω and 0.47μF respectively. The closer R and C are to the load, the better the anti-interference effect is, as shown in Figure 3.
(2) For DC load, connect a freewheeling diode in parallel at both ends of the load, as shown in Figure 4. The diode should also be close to the load, and the reverse withstand voltage of the diode should be 4 times the load voltage.

When the controller switches the output, it is best to adopt the anti-interference measures shown in Figure 3 (AC load) and Figure 4 (DC load) regardless of whether the controller itself has anti-interference measures. In situations where large interference is generated during switching, the AC load can use a bidirectional thyristor output module. The method of using an intermediate relay in the control panel to drive the load is very effective. For the anti-interference technology of electronic equipment, the main principle is to suppress the interference source. The interference of the output signal of the PLC programmable controller can be introduced into the earth through good grounding, thereby reducing the impact of interference.